Squaric acid
Squaric acid, also called quadratic acid, because its four carbon atoms approximately form a square, is an organic compound with chemical formula C4H2O4.
The conjugate base of squaric acid is the hydrogensquarate anion C4HO4; and the conjugate base of the hydrogensquarate anion is the divalent squarate anion C4O4. This is one of the oxocarbon anions, which consist only of carbon and oxygen.